The first close-up pictures of the massive asteroid Vesta reveal a northern hemisphere littered with craters — including a trio nicknamed "Snowman" — and a smoother southern half, researchers reported Monday.

Running along the asteroid's equator are deep grooves — a surprise to scientists who did not expect to see such features.

"We're seeing quite a varied surface," said chief scientist Christopher Russell of the University of California, Los Angeles. The images were taken by NASA's Dawn spacecraft, which began orbiting the 330-mile-wide rocky body last month and beaming back incredible surface details that the team is only beginning to pore over. It's the first time that Vesta has been viewed up close. Until now, it has only been photographed from afar.

Since entering orbit, Dawn has taken more than 500 pictures, while refining its path and inching ever closer to the surface to get a better view. The probe will officially start collecting science data next week once it is 1,700 miles from the surface. It will get as close as 110 miles while it orbits Vesta for a year.

Vesta's southern section is dominated by a giant crater, the result of a collision eons ago that's believed to have pelted Earth with numerous meteorites, or broken off pieces of asteroids. The northern side is filled with older craters including three that scientists dubbed "Snowman."

Vesta is "so rich in features" that it will keep scientists busy for years, said Holger Sierks, of the Max Planck Society in Germany, who helps operate the camera.

Currently some 117 million miles from Earth, Vesta is the second-largest resident of the asteroid belt, a zone between Mars and Jupiter filled with hundreds of thousands of space rocks orbiting the sun. The belt formed some 4.5 billion years ago around the same time and under similar conditions as Earth and the inner planets.

It's thought that larger chunks such as Vesta could have merged into planets had they not been foiled by Jupiter's gravity. Despite being denied planethood, asteroids are of interest to researchers because they date back to the early solar system.

Powered by ion propulsion instead of conventional rocket fuel, Dawn slid around Vesta on July 15 after a 1.7 billion-mile cruise. Most orbit insertions are tricky because a speeding craft has to slow down or risk overshooting its target.

Since Dawn has been traveling slow relative to Vesta, the orbit capture was a ho-hum event.

Launched in 2007, Dawn is the first mission to explore Vesta and Ceres, the two largest members of the asteroid belt. It's also the largest interplanetary probe launched by NASA, measuring 64 feet tip to tip with its solar panels unfurled.

Though the $466 million project was conceived long before the United States decided to send astronauts to an asteroid by 2025, the data gathered by Dawn should help future manned missions.

After a year, Dawn will move on to Ceres, where it will arrive in 2015.

Unlike dry and rocky Vesta, Ceres is icy and may have frost-covered poles. Due to the possible presence of frozen water, Dawn will not be able to venture as close to Ceres' surface for fear of contaminating the asteroid.

The team does not plan to post raw images online as other NASA missions have done. Instead, there will be just one picture released daily.

NASA PRE-RELEASE INFO

Even before entering into orbit around the asteroid Vesta in the middle of July, NASA's Dawn spacecraft had discovered a mysterious dark spot at the asteroid's equator. Since Dawn has been taking numerous images of Vesta.

Dawn has found a dry, rocky world littered with impact craters floating in the asteroid belt between the orbits of Mars and Jupiter. It is thought the asteroid belt consists of the remnants of material that might have formed into a planet had the gravitational effects of Jupiter had prevented it, driving the material apart instead of allowing it to coalesce. Vesta might well have grown to a rocky world much like Earth or Mars had this not happened. As it is, Vesta is 330 miles across and is classified as a "protoplanet."

A number of spectacular images of Vesta are due to be released by NASA. Preliminary results suggest a number of processes have happened on Vesta during the years of its early formation.

The Dawn spacecraft will spend a year orbiting Vesta, studying it with a suite of instruments, attempting to understand how the early solar system was formed. Then the ion engines will blast the Dawn out of orbit around Vesta and will take the spacecraft to Ceres, the largest asteroid in the main belt, for a year or more of study of that celestial body starting in 2015. Ceres measures 590 miles in diameter and is thought to contain ice, making it a different type of world than Vesta.

Dawn is part of NASA's Discovery series of spacecraft, costing $466 million. Dawn was launched on Sept. 27, 2007, on a Delta II rocket, and used an ion engine, with the help of a gravity assist maneuver at Mars to make the nearly four year, over 1.7 billion mile voyage to Vesta. Dawn will study Vesta from a variety of orbits, including one at a distance of just 110 miles from the asteroid's surface.

The images taken of Vesta hitherto have been done to calibrate the cameras and other scientific instruments on board Dawn. The scientific study of Vesta now begins in earnest.